Ramos did not seem interested in the large salary offer from Al-Ittihad, he preferred Sevilla as a club at the start of his professional career. Ramos still has the opportunity to play in the top league even though he is 37 years old. Sevilla is also one of the clubs competing in the UCL this season.
Sergio Ramos is known for making records with his attitude and red cards. I could imagine being carded from Saudi Arabia altogether if he play there

As a free agent, he didn't receive an offer from Inter Miami, a Turkish team was looking for him too and offered €11m a year but he refused. He didn't care also about Al-Ittihad offer of €20m a year and preferred to join Sevilla which only can afford €1m which he accepted. Normally after medical visit he will be a Sevilla defender again after 18 years from his departure there.
It is very rare for a player to refuse a large salary offer. It is possible that Ramos chose Sevilla because of encouragement from his family who did not want to move outside Europe. Yes, we all know that Ramos was formed at Sevilla, and became a legend for Real Madrid. It could be that his return is a form of appreciation and gratitude for his old club, that makes sense too.
Sevilla is currently slumped at the bottom of the standings, they have not been able to get a single point so far. Ramos is expected to be able to bring Sevilla to winning ways, wishing Ramos the best for the rest of his career.